fix(format): refuse path-like group and dataset names

FileWriter writes the root group plus one level of groups; it has no way
to create intermediate groups. create_group("a/b") therefore stored a
single link literally named "a/b", which no HDF5 reader can resolve
(h5py: "component not found"). Nesting would mean restructuring the
writer's layout around a group tree, so for now finish() rejects any
group, dataset or external-link name that is empty, "." or contains '/'.
Attribute names may still contain '/'.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/file_writer.rs

@@ -63,6 +63,19 @@ fn build_paged_superblock_extension(page_size: u32) -> Result<Vec<u8>, FormatErr
     w.serialize()
 }
 
+/// A group or dataset name must be one path component: not empty, not ".",
+/// and without '/'. `FileWriter` writes a root group plus one level of
+/// groups, and cannot create intermediate groups for a path.
+fn check_link_name(name: &str) -> Result<(), FormatError> {
+    if name.is_empty() || name == "." || name.contains('/') {
+        return Err(FormatError::SerializationError(format!(
+            "invalid object name {name:?}: names must be a single path component \
+             (FileWriter does not create nested groups)"
+        )));
+    }
+    Ok(())
+}
+
 /// Threshold for switching from compact (inline) to dense attribute storage.
 const DENSE_ATTR_THRESHOLD: usize = 8;
 
@@ -1137,6 +1150,19 @@ impl FileWriter {
             })
         };
 
+        // Every name becomes a single link in its parent group. The writer
+        // has no nested groups, so a path like "a/b" would be stored as one
+        // link literally named "a/b" — which no HDF5 reader can resolve.
+        let root_names = self.root_datasets.iter().map(|d| d.name.as_str());
+        let group_names = self.groups.iter().flat_map(|g| {
+            core::iter::once(g.name.as_str())
+                .chain(g.datasets.iter().map(|d| d.name.as_str()))
+                .chain(g.external_links.iter().map(|l| l.0.as_str()))
+        });
+        for name in root_names.chain(group_names) {
+            check_link_name(name)?;
+        }
+
         let mut all_ds: Vec<DsFlat> = Vec::new();
         let mut groups: Vec<GrpFlat> = Vec::new();
         let mut root_ds_indices: Vec<usize> = Vec::new();